impregnable castle; impregnable fortress
Figurative or literal: a castle so strong it cannot be taken. Often used metaphorically for an unassailable position or stronghold.
This fortress is truly an impregnable castle.
彼の守備は金城のようだ。
His defense is like an impregnable fortress.
inner citadel; inner castle
Historical term for the innermost fortified area of a Japanese castle, the last defensive stronghold.
The inner citadel is the most fortified part of the castle.
Golden Castle; Nagoya Castle
Nickname for Nagoya Castle, referring to its golden shachihoko ornaments. Used in tourism and historical contexts.
The Golden Castle is a symbol of Nagoya.
They turned the mountain castle into an impregnable stronghold to prepare against enemy invasion.
本丸 is the standard castle-terminology word for the innermost bailey, whereas 金城 is a more literary or figurative term for the same inner stronghold.
From 金 (kin, 'gold' or 'metal') and 城 (jō, 'castle'), literally 'golden castle' or 'metal castle'. The sense of 'impregnable' comes from the idea of a castle as strong as metal.